Presently available continuously variable transmissions (CVTs) have the ability to vary speed ratios in infinitesimal steps over a wide range of speeds to permit automotive engines to be operated at the most efficient speed for the load conditions and to be rapidly and smoothly shifted to a ratio that allows maximum engine power when required. However, use of the CVT has been severely limited by poor power handling capacity and relatively low efficiency of many CVTs which negates most of the advantages in improving the operating efficiency of automotive engines. For high power ratings required for automotive applications, four types of CVTs are available. These are the hydrostatic, the V-belt, the traction, and the flat belt types.
The hydrostatic CVT, such as those manufactured by the Eaton Corporation, designated as Eaton Hydrostatic Transmissions Model 33 through Model 76, consist of a variable displacement hydraulic pump driving a fixed displacement hydraulic motor. These are commercially available in capacities up to 200 HP for use on heavy construction equipment and provide full load operating efficiencies of over 85%. The speed ratio is continuously variable from full forward to full reverse. The generally large size requirements, high noise, low efficiency and cost make this type unsuitable for automotive applications.
The V-belt type of CVT, such as one developed in Holland by van Doorne Transmissie, which has been utilized in Fiat, Renault, Subaru and Ford automobiles, is similar to a conventional V-belt drive except that the relatively complex, multi-segmented belt is made wide so that the faces of the pulleys on which it rides can be moved together or apart. Moving the faces of one pulley together and the faces of the other pulley apart causes the belt to ride at a larger radius on the first pulley and at a smaller radius on the second pulley thus causing the speed ratio of the first and second pulleys to increase. This type of transmission is capable of operating at speed ratios in excess of 4:1, but it has no reverse capability. It has been applied to relatively low powered automobiles at ratings of up to 60 HP with efficiencies reported to approach 90%.
The traction type of transmission, such as described in U.S. Pat. No. 3,822,610 by Erban, typically consists of hardened steel rollers operating against a pair of toroidal discs. An extremely high contact force allows the rollers to transmit considerable power without slippage. Tilting the rollers changes the drive ratio between the discs. The device is capable of an efficiency of over 98% at full forward and 80% in full reverse with power ratings of several hundred horsepower being possible. However, the high cost of the required high strength materials limits its applicability. Also, continuous operation of the traction type of transmission at a constant speed ratio often leads to wear of the toroidal discs and subsequent control difficulties.
The flat belt type of transmission such as described in U.S. Pat. No. 4,295,836 by Emerson L. Kumm, has various advantages relative to the other variable speed transmissions as described above including higher efficiencies over a wider speed ratio range and a more compact size for a given power with a relatively low cost belt. However, it does not have a reverse capability, and, like V-belt CVTs, its minimum output speed is above zero, increasing with increases in the input speed.
One known example of an attempt to improve the use of a CVT by combining it with a gear mechanism is found in U.S. Pat. No. 3,527,119. This device, however, utilizes a unidirectional variable speed motor which can be selectively coupled to one of two plural paths to control the relative speeds. Each path can be stopped separately to achieve shifting without requiring braking but cannot be shifted under load. The variable speed drive is non-reversible and the device has only limited use for automotive applications.
One known example of measuring torques is shown in U.S. Pat. No. 3,253,658. Here, the torque of a shaft is obtained rather than the pulley belt slip which is used to measure torque in the subject transmission. The belt slip, while giving a measure of the pulley or shaft torque, gives a more critical and vital measurement of the pulley operation; i.e., the belt efficiency and durability.